Shielded printed circuit board

ABSTRACT

An improved shielding assembly for a circuit board. The assembly includes a multi-layer circuit board, each layer of which has ground planes around the perimeter thereof. An edge plating on the circuit board connects the ground planes to one another. A number of tab holes are also in the board, and the shielding assembly is formed as a conductive box which is connected to the tab holes. A shielded area is formed within the shielding assembly. The shielding assembly is preferably formed by etching fold locations and folding, and by etching tabs to be formed.

FIELD OF THE INVENTION

The present invention relates to a shielded printed circuit board whichprovides maximum shielding to the circuit on the board and which isformed in a completely new way.

BACKGROUND OF THE INVENTION

Electronics have become more miniaturized and sophisticated in recentyears. However, along with sophistication and miniaturization comes thenecessity for better noise shielding.

Noise immunity is extremely important in printed circuit boards for usein communication equipment. Crucial parts of the board must be shieldedagainst noise and RF interference. Shielding also provides some immunityfrom unauthorized reception by sophisticated surveillance equipment. Theperfect shield might be a grounded, sealed box, covering the whole unit.However, this would not allow for power signal access and thereforewould not be a usable assembly.

Another necessary attribute of modern circuit boards are that they be ofthe multi-layer type, in order to facilitate multiple connectionsbetween various components. Multi-layer circuit boards have a pluralityof layers, each of which is formed with a number of traces. Thecomponents on the board face re connected with one of the layers, andtherefore connected by the traces on that layer and the traces on noother layers. Each of these layers also has a ground plane.

One standard way of shielding against noise has been to provide a groundplane. Such a ground plane enables ready connection to "ground" which intheory has no noise. However, the way in which the ground planes weretreated in the prior art was relatively deficient.

One attempted RF shielded enclosure is shown in U.S. Pat. No. 4,658,334to McSparran. This technique provides an RF shield around the circuitboard at top and bottom and also show putting an RF shield 104 on theedges of the multi-layer board. The RF shield on the edges, however,does not necessarily make good contact with these edges. Moreover, theshielding enclosure of McSparran is complicated as it must be located ontop and bottom and must be screwed and unscrewed in order to reach theboard.

Another attempted solution is shown in U.S. Pat. No. 3,179,913 toMittler. FIG. 21 of Mittler shows a multi-layer matrix board which usesedges formed of a ground plane in order to attempt to shield the device.A plurality of these devices are located in racks, and multiple layersof the board are held together by screws. Therefore this system is hardto use. It is an object of the present invention to provide aneasy-to-assemble and use system which enables ready access to thecomponents on the circuit board but yet provides a maximum shielding forthe circuit board.

SUMMARY OF THE INVENTION

The present invention proposes a special multi-layer circuit board. Eachlayer except the top and bottom layer have edges which define a groundplane for the component traces on the board. The top and bottom layersare used as connecting planes and include components, connectors and thelike. The board also has a shielded area within which the sensitivecomponents are located. The shielded layer is formed with a number ofshielding holes that are plated-through, between every layer of themultiple layers of the board. Each of the shielding holes are used tohold tabs of a shielding assembly.

The shielding assembly is formed of tin or nickel-plated brass, or anyother. Material to which solder can easily be applied having extendingtabs, each connected to one of the shielding holes. The shieldcompletely surrounds the shielding area. The shielding assembly can beformed with at least one hole through which a status of the componentscan be determined, an adjustment can be made using an appropriate tool,and/or connection to the circuits can be performed.

The conductive shielding assembly is connected to the ground planes ofall of the multi-layer boards of the assembly. Moreover, an edge platingis provided between the multi-layer boards in order ensure maximumconnection between the ground planes and to eliminate "ground loops".Finally, an external connector is provided, removed from the shieldedarea. This external connector is connected through the multi-layerboards to a filter and isolation area. The signals entering the shieldedarea are isolated, for instance by optoisolators, and filtered in orderto ensure that noise on these signals does not correspondingly enter theshielded area. This also ensures that a ground isolation is maintainedbetween the various signals and the components.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages will now be described with reference to theaccompanying drawings wherein:

FIG. 1 shows an embodiment of the invention including all of the variouscomponents located thereon, from a top view including two partialbroken-away portions showing components underneath;

FIG. 2 shows a side view of this embodiment;

FIG. 3 shows a closeup of the edge of the board of this embodiment;

FIGS. 4A and 4B illustrate perspective views of first and secondembodiments of the shielding assembly in its open position;

FIG. 4C shows a close up of the folding and bonding according to thefirst embodiment;

FIG. 5 shows a connection between the shielding assembly and the circuitboard;

FIGS. 6A-6C show steps of producing the shielding assembly of thepresent invention; and

FIG. 6D shows a representative layout of one of the interior multi-layerboards of the present invention.

DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT

A presently preferred embodiment will now be described with reference tothe accompanying drawings. FIG. 1 shows a top view of the preferredembodiment of this invention. This embodiment is formed of a shielded,multi-layer circuit board including a shielding assembly and otherassociated structure. The multi-layer board 100 is formed with aplurality of shielding holes 102. For clarity of the drawing, not allshielding holes 102 are labelled, but some representative ones of theshielding holes are labelled. Shielding holes 102 receive tabs 104,shown in FIGS. 2 and 5, of a shielding assembly 110.

Shielding assembly 110 is formed of a rectangular box of tin ornickel-plated brass open at the bottom 440 and closed by top 410 andhaving corner edges 112, and sides 114 and 142. FIG. 4A shows a firstembodiment of shielding assembly 110 in a partially cut away view. Thisfirst embodiment forms the box as a two piece shielding unit including aperimeter 442 and a top or fence 410. The two pieces are attachedtogether using metal tape 402. The perimeter 442 is formed by bendingsheet metal to form four side pieces 404 formed as a rectangle, and toform a lip 406 extending around the top portion (the portion that willbe closed) of the rectangle. Before the fence 410 is placed on theperimeter, therefore, it has an open area 408 bonded by a lip 406 whichextends into the open area.

In order to close the box, top 410 is placed on the box, and secured onthe box around the four ends with metal tape 402. The metal tape isadhered to the top 410 and each of the four sides 404. A close up of thebonding is shown in FIG. 4C and shows how top 410 rests on lip 406.

One advantage of this embodiment is that the top 410 can be removed, ifnecessary, to obtain access to the components by cutting the metal tape.However, this embodiment has the disadvantage of requiring assembly ofthe device. The second embodiment of FIG. 4B forms a shield of a onepiece construction. The top portion of the shield 420 is attached toside pieces 422, by bending these side pieces away from the top portionof the shield. If necessary, spot welds may be located at the corners424.

FIG. 1 shows connector 130 disposed spaced from the shielding assembly110, through which input and output connections with the circuit boardare made. Signals to and from connector 130 are coupled throughconnector 130 to traces formed in the multi-layer board beneathconnector 130. These signals are connected via these traces to isolatorfilter bank 140, which is located within shielding assembly 110 at alocation most closely adjacent to connector 130. Isolator filter bank140 isolates between the input and output signals using, for example,optoisolator assemblies and noise filtration techniques. Once isolatedand filtered, these signals are connected through the multi-layer tracesto the circuitry within shielding assembly 110. This circuitry is showngenerally as 160 in FIG. 1, it being understood that any type ofcircuitry could be used therein.

Circuitry may also provided on the board 100 outside of the shieldingassembly 110.

Various holes may also be formed in the shielding assembly as shown inFIG. 1. One hole that may be formed is hole 122, adapted to allow statusLED 120 to be seen through it. In addition, signal holes as shown byelements 170 and 172. These holes allow connections of optical cabledirectly to optically sensitive elements within the device.

FIG. 2 shows a side view of the circuit board, as seen from thedirection 2 in FIG. 1. This side view shows the conductive tape 402, andside 404, and shows the connector 130 as spaced from the shielded area110. The edge of circuit board 100 is shown as being plated over at allareas except for notch areas 220. Each notch area 220, is of the lengthof 1/4 of a wavelength. At all other areas, the ground planes of theinterior layers of the multi-layer board are connected to one another byedge plating the circuit board.

In order to facilitate explanation of this edge connection better, arepresentative layer of the multi-layer board is shown in FIG. 6. Theperimeter of the entire layer is a ground plane 230. This ground plane230 is connected to the ground of the system, and surrounds a connectionarea 240 within which traces to connect various points on the circuitboard to various other points are located. If a particular hole 242 isbeing used in the particular layer, it will be plated around, andconnected via a trace 246 to another hole 244.

The edges 232 of ground area 230 are exposed at the edge of the circuitboard which is plated over by plating 205 shown in FIG. 2. The shieldedarea of the board will also be surrounded by plated-through holes 102 oneach layer of the board.

FIG. 3 shows a close-up of area 3 in FIG. 2. This close-up shows theplated area 205 which is covered by plating, and which makes electricalconnection with a plurality of layers. The notch 220 is also shown inFIG. 3. FIG. 3 shows a multi-layer board which has three interiorlayers. Each of these interior layers provides edge surfaces 232 of theground area 230. These edge surfaces are connected together by plating205 which itself is connected to the tabs 104 of the shielding assembly110. Therefore, the shielding assembly 110 is also advantageouslygrounded to these ground planes.

This grounding technique of grounding at many points prevents thephenomenon known in the art as "ground loops" from occurring. Groundloops occur when a ground potential is connected at one point, and at along distance from that one point. Because no ground plane has a perfectzero resistance, there will be some voltage drop across the resistancealong the ground plane. The voltage drop will cause the ground potentialat one spot to be different from the voltage at other spots, thuscausing a voltage potential difference. If the voltage at one spot isconsidered ground, and at the other spot is considered ground, therewill two different voltage potentials, both of which are ground. This isknown as a ground loop. The shielding techniques and ground connectionsof the present invention tend to ensure that such ground loops will beminimized.

FIG. 5 shows a detail of the connection between the tabs 104 of theshielding assembly 110 and the holes 102 formed in circuit board 100.

The fabrication of the circuit board is an especially important part ofthe present invention. An object of the invention is to make a circuitboard which has optimal characteristics and yet is still easy toassemble. In order to do this, it is desirable to fabricate the shieldswith a high degree of accuracy, so that the tabs 104 of the shield areaccurately aligned with the holes 102 making it a simple matter toinsert these shields. As stated previously, the tabs are located with adistance of a quarter wavelength between each tab. The plated-throughholes are also relatively small, thereby making the accuracy of theactual shape of the box that forms the shield very important.

However, the way in which the prior art has folded boxes for shields isalso relatively inaccurate.

The assembly technique of the shield of the present invention has beenspecially formulated in recognition of this problem.

The assembly technique of the present invention begins with a flat sheetof appropriate metal, either tin or nickel-plated brass. This flat sheetis shown in FIG. 6A. Before any folding, this sheet is chemically etchedby forming a computer aided drafting artwork template. This computeraided drafting artwork template shows the bends and folds which arenecessary to form the box shown in FIG. 4A or 4B. For simplicity, thedevice formed will be the one shown in FIG. 4B. The computer aideddrafting template is used for form grooves on flat sheet 600, includinggrooves 602, 604, 606 and 608. These grooves are precisely located atthe precise locations which are necessary in order for the box, whenfolded to precisely fit onto the circuit board. At this point, however,no tabs are yet formed. The tabs are formed as shown in FIG. 6B whichshows only a section of FIG. 6A below the line 6B, 6B in FIG. 6A. Thissection shows the second phase of the scoring which is done using thecomputer aided drafting artwork. This second etching process etches thehatched area shown in FIG. 6B to form the plurality of tabs 104.

Since the hatched part is removed by etching in FIG. 6B, thisequivalently leaves tabs 104 extending. Also, the portion labeled 615 isremoved so that an accurate fold can be made.

The result after folding the assembly of FIG. 6B is shown in FIG. 6C.The edge 620 of fold 604 is abutted against the edge 622 of fold 606.Similarly, the edge 624 abuts against edge 626. This equivalently formsthe box structure shown in FIG. 6C with tabs 104 extending downward, andhaving exact, and more importantly highly reproducible, locations offolds and tabs. Therefore, all boxes will be the same as all others.

Finally, the purpose of the breaks in the edge plating will now bedescribed. In order to plate the edges of the circuit board, it isnecessary to dip the circuit board to allow these edges to come incontact with a plating solution. In order to do this, however, thecircuit board must be held. The locations at which the circuit board isheld therefore would not be plated with a solution.

It has been found by the inventors that forming each break such that ithas a length of substantially a quarter of a wavelength of an operatingfrequency minimizes the noise allowed by the breaks.

Although only a few embodiments have been described above, those ofordinary skill in the art will certainly understand that manymodifications are possible in this embodiment without departingmaterially from the teachings given herein. For example, although metaltape has been described as being the closure material, other closuresare possible, including welds or solder. Although only one externalconnector has been shown, many such connectors may be provided. Theseand other modifications which would be appreciated by those havingordinary skill in the art and are intended to be encompassed within thefollowing claims.

What is claimed is:
 1. A shielded circuitry assembly, comprising:amulti-layer circuit board, having:a) a plurality of internal layers,each with a ground plane defined around a perimeter of a wiring surfacethereof, each said ground plane having exposed edge surfaces, b) a toplayer, adapted to receive electrical components, c) a plurality of tabthrough-holes, extending through the entire circuit board, includingthrough the internal layers, and the top layer, and in contact with eachsaid ground plane of each said internal layer, d) an edge plating,connecting said exposed edge surfaces of said ground planes to oneanother, and extending around at least a majority of a perimeter of saidmulti-layer circuit board, and e) a plurality of componentthrough-holes, extending through the circuit board, enabling connectionwith wiring traces on the internal layers; and a shielding assembly,coupled to said top layer of said multi-layer circuit board, and havingtabs which are located in said tab through-holes and make electricalcontact with said each ground layer, said shielding assembly having aperimeter with inner surfaces defining an inner shielded areatherewithin, and having a top surface of a similar size to saidperimeter for closing said inner shielded area beneath said top surface.2. An assembly as in claim 1 wherein said edge plating is formed withbreakers breaks, each said break having a length which is substantiallya wavelength of an operating frequency divided by four.
 3. An assemblyas in claim 1 wherein said top surface has at least one hole therein,said hole enabling communication with circuitry on said top layer.
 4. Anassembly as in claim 1, wherein said top surface is integral with saidsurfaces defining said perimeter.
 5. An assembly as in claim 1 whereinsaid top surface is attached to said surfaces defining said innershielded area, and further comprising a conductive substance forattaching said top surface to said perimeter.
 6. An assembly as in claim5 further comprising a connector, spaced from said shielding assembly,for conducting signals to and from circuitry within said shieldingassembly; means for filtering signals from said connector, saidfiltering means located within said shielded area at a position closestto said connector.
 7. An assembly as in claim 6 wherein said filteringmeans also includes means for optically isolating said signals fromsignals within said shielded area.
 8. An assembly as in claim 4, whereinsaid conducting substance comprises conducting metal tape, and whereinsaid circuit board also has a surface for accepting circuitry outside ofsaid shielded area.
 9. A shielded circuitry assembly, comprising:amulti-layer circuit board, having:a) a plurality of internal layers,each with a ground plane which has exposed edge surfaces, b) a toplayer, adapted to receive electrical components, c) a plurality of tabthrough-holes, extending through the internal layers, and the top layer,and in contact with each said ground plane of each said internal layer,d) an edge plating, connecting said exposed edge surfaces of said groundplanes to one another, and extending around a perimeter of saidmulti-layer board, and e) a plurality of component through-holes,extending through the circuit boards, enabling connection with wiringtraces on the internal layers; and a shielding assembly, including acover, and coupled to said top layer of said multi-layer circuit board,and having tabs which are located in said tab through-holes and makeelectrical contact with said each ground layer, said shielding assemblyhaving a perimeter with inner surfaces defining an inner shielded areatherewithin, a top portion of said perimeter forming a cover receivingsurface, extending around said perimeter and facing upwards, andincluding a top surface, coupled to said cover receiving surface anbiased thereagainst and means for connecting said cover to said coverreceiving surface said means being electrically conductive.
 10. Anassembly as in claim 9 wherein said edge plating is formed with breakersbreaks, each said break having a length which is substantially awavelength of an operating frequency divided by four.
 11. An assembly asin claim 9 wherein said top surface is formed with at least one holetherein, said hole enabling communication with circuitry on said toplayer.
 12. An assembly as in claim 9 wherein said connecting means isformed of conducting metal tape.
 13. An assembly as in claim 9 furthercomprising a connector, spaced from said shielding assembly, forconducting signals to and from circuitry within said shielding assembly.14. An assembly as in claim 13 further comprising means for filteringsignals from said connector, said filtering means located within saidshielded area at a position closest to said connector.
 15. An assemblyas in claim 14 wherein said filtering means also includes means foroptically isolating said signals from signals within said shielded area.16. An assembly as in claim 15 wherein said circuit board also has asurface for accepting circuitry outside of said shielded area.
 17. Ashielded multi-layer circuit board, comprising:a) a plurality ofinternal layers, each with a ground plane defined around a perimeter ofa wiring surface thereof, each said ground plane having exposed edgesurfaces, b) a top layer, adapted to receive electrical components, c) aplurality of tab through-holes, formed through the entire circuit board,including through the internal layers, and the top layer, and each tabthrough-hole in contact with each said ground plane of each saidinternal layer, d) an edge plating, connecting said exposed edgesurfaces of said ground planes to one another, and extending around aperimeter of said multi-layer board, and e) a plurality of componentthrough-holes, formed through the circuit board, enabling connectionwith wiring traces on the internal layers; and a shielding assembly,coupled to said top layer of said multi-layer circuit board, and havingtabs which are located in said tab through-holes and make electricalcontact with said each ground layer, said shielding assembly having aperimeter with inner surfaces defining an inner shielded areatherewithin.